Field of the Invention
The present invention relates to a localized surface plasmon resonance (LSPR) sensing system with particles arranged in an anisotropic period, especially to a LSPR sensing system in which the spectra of two orthogonal polarizations of transmitted light or reflected light are a bit different due to metal nanoparticles arranged in an anisotropic period in a metal nanoparticle layer of the test specimen. Thus a signal of phase difference with a quite narrow bandwidth is generated and can be measured by ellipsometry. Therefore the figure of merit of the sensing system is significantly improved.
Description of Related Art
The LSPR is a collective oscillation of free electrons in metal nanoparticles when excited by electromagnetic waves. The LSPR induces peaks or troughs in spectra of absorption, scattering, transmittance or reflectance at the resonance frequency. The resonance frequency of metal nanoparticles will shift due to delicate change of refractive index of the environment which can be caused by binding of molecules on nanoparticles or change of solution density of chemical substances, etc. Thus the refractive index changes can be learned by monitoring the spectral shift of resonance frequency. The LSPR technique with advantages of high sensitivity and real-time detection has been widely used in chemical and biological sensors. The sensitivity of sensor is defined as the spectral shift divided by a refractive index change. In addition to the sensitivity, the capability of sensor is also concerned with the full width at half maximum (FWHM) of the peak/trough. A figure of merit (FOM) defined as the sensitivity divided by the FWHM is widely used to characterize the sensor performance. It can be expected that the sensor with narrower bandwidth has better quality/performance among sensors with the same sensitivity.
The LSPR sensors offer small FOM typically ranging from 1 to 2 owing to the broad line shape of LSPR. In recent years, many methods for restraining the FWHM to improve the FOM have been proposed. Leif J. Sherry et al. (Nano Lett. 5, 2034 (2005)) observed a higher order mode of nanocubes excited by the contact of the substrate. The mode possesses a FWHM narrower than the dipole mode, and thus resulting in a higher FOM. Peter Offermans et al. (ACS Nano 5, 5151 (2011)) demonstrated an enhanced FOM of sensor of periodic arranged nanoparticles. The coupling of Wood-Rayleigh anomaly and the LSPR restrains the FWHM of the sensor. Currently, most of the proposed methods are based on detection of light intensity. However, according to the research of Andrei V. Kabashin et al. (Opt. Express 17, 21191 (2009)), phase detection algorithms with the advantages of higher signal-to-noise ratios and better sensitivities give the possibility to achieve lower detection limits.
A novel phase detection algorithm with wavelength interrogation is proposed by Kristof Lodewijks et al. (Nano Lett. 12, 1655 (2012)). They designed test specimen which are composed of a gold thin film, a dielectric layer and a nanoparticle layer. In the instrument, the resonant frequency of S polarization and P polarization is separated by oblique incidence and thus a phase difference is generated between the two polarization states. The bandwidth of the phase signal measured by ellipsometry is narrower than the bandwidth of the reflectance so that the FOM is increased 6.1 times. However, the fabrication process of the test specimens is more complicated than general LSPR test specimen due to the additional gold thin film and the dielectric layer disposed under the nanoparticle layer. Thus the manufacturing cost is increased. Moreover, its optical path is incident obliquely and is achieved by a rotary arm or more complicated optical design. The above shortcomings hinder the commercialization of the technique. Thus there is room for improvement and a need to provide a novel LSPR sensing system that overcomes the above shortcomings.